The present invention relates to a method of preparing dentifrice abrasives, and more particularly to dicalcium phosphate dihydrate with improved stability.
Dicalcium orthophosphate dihydrate (CaHPO.sub.4.2H.sub.2 O) that has been stabilized against spontaneous hydrolysis and/or decomposition with a small amount of tetrasodium pyrophosphate in accordance with the processes such as those described by Moss et al in U.S. Pat. No. 2,287,699 or with trimagnesium orthophosphate and the like has been utilized in dental preparations for many years. Indeed, dicalcium phosphate dihydrate frequently is stabilized against spontaneous hydrolysis and/or decomposition with a small amount of both tetrasodium pyrophosphate and trimagnesium phosphate in dental preparations.
Furthermore, as is known to those skilled in the art, dentifrice formulations using dicalcium phosphate dihydrate frequently contain sodium or potassium monofluorophosphate as a source of fluoride ion to inhibit or retard the formation of dental caries. Thus, the use of dicalcium phosphate dihydrate with sodium or potassium monofluorophosphate and stabilized against spontaneous hydrolysis and/or decomposition with tetrasodium pyrophosphate and/or trimagnesium phosphate, either with or without other polishing agents, are well known to those skilled in the art.
Although satisfactory results are obtained using the dental formulations as set forth above, it has been found that over a period of time the soluble fluoride is lost from the dental formulations. For example, it has been found that dental formulations containing dicalcium phosphate dihydrate stabilized with tetrasodium pyrophosphate at up to 1 weight percent by weight P.sub.2 O.sub.5, as pyrophosphate, or about 2 weight percent trimagnesium phosphate octahydrate, and sufficient sodium monofluorophosphate to provide about 1,000 parts per million soluble fluoride will lose a substantial amount of the soluble fluoride after prolonged storage. Only a small improvement is seen when both trimagnesium phosphate and tetrasodium pyrophosphate are used together with respect to soluble fluoride stability.
Although Applicants do not wish to be bound by any particular theory, it is believed that the loss of soluble fluoride in the formulation is related to the hydrolytic instability of the dicalcium phosphate dihydrate. It is believed that fluoride ion catalyzes the formation of calcium hydroxyapatite which then reacts with the soluble fluoride to form water-insoluble calcium fluoroapatite and/or calcium fluoride. Hence, it can be seen that improved fluoride stability of dicalcium phosphate dihydrate may improve stability against spontaneous hydrolysis and/or decomposition.
In any event, it can be seen that there is a need for a method of preparing dicalcium phosphate dihydrate which will provide greater amounts of soluble fluoride after prolonged storage. Now, a method to meet this need is provided.